Back in the net-metering days... power-system modeling (and related communication) was simpler. Financially, a PV energy unit (kWh) was an energy unit. But now, some PV energy units are 1) very valuable (the ones that directly serve loads (in a high-electric-rate area)), 2) pretty valuable (that go into a battery and serve loads later), 3) near worthless (that are exported to the grid for wholesale credit), or 4) straight-up worthless (that are "vaporized" in a non-export setup, when loads are already served and the battery is full).

To have a fighting chance at a good $ investment, we need to maximize the PV energy units that are used on site. A big part of that is moving the big flexible loads, to better match when PV power is available. ...I write "flexible" because this needs to be done in a way that doesn't annoy occupants.

...This adds complication and prospect wariness. Another option is to add a lot more PV and battery — but that's typically a lot more expensive than flexible-load control. ...At least versus the low-hanging load-control fruit (like car charging at a site where the car is parked and plugged in most days).

Another option is to lie in our modeling (or let SaaS software lie for us)... don't bring up load control, and just assume that the occupants will do it. Or something like that.

I'm trying to simplify where practical, but I can't not geek with prospects. Maybe a niche will be technically-minded prospects that are down to crawl around in rat holes with me.

A very cool prospect in Berkeley, referred by a nearby family I previously installed for, recently received a geek email. ...And she replied positively, with helpful technical feedback. Hot damn, thank you.

Her (residential) PG&E interval data is in 15-minute increments. As always, I disaggregated that and (AMY) PV production estimates to 5-minute intervals for battery modeling.

Here's my email to her:

If you decide to move forward with me but I don't get the system installed / permitted until early next year (post 30% federal tax credit)... I'll take a significant part of the hit thru a labor-cost discount. Also, I expect PV + battery equipment costs to come down once the tax credit is gone.

I don't know how game you are to geek with me on details... please let me know if the writing below is way TMI. (The modeling was much simpler back when there was net metering (when PV-system owners got retail credit for extra PV power exported to the grid).)

For now, I'm assuming a 5.5 kilowatt (kw) PV system (13 PV modules) and a 16 kilowatt-hour (kwh) battery (this is a typical combo). My first-pass analysis will (unrealistically) assume full sunlight. If the financials work for you, I'll then come by to do a site inspection / shade analysis.

For a sample billing period: 2024-07-11 thru 2024-08-08...

Your usage was 809 kwh.

My first-pass modeling assumes that 267 kwh (33%) would be handled directly by the PV system. 282 kwh (35%) would be handled by the battery (using extra stored PV power.

...261 kwh (32%) would still come from the electric grid. Which I think is too much.

You noted that you have EV charging, a hot tub, and electric heating. These big loads running for 30+ minutes at a time... account for 71% of your usage. A problem is that they quickly deplete your battery, and then they're pulling from the grid.

Meanwhile, more than half of your PV power (248 out of 515 kwh) would either get exported (for peanuts compensation) or lost (if we go the zero-export route).

To make the PV + battery system a much better $ investment... ideally we "flatten out" the big loads. In other words, charge the car dynamically here and there with spare PV power, instead of maximum charging all at once. And potentially pre-heat your house and hot tub (to the extent comfortable / practical) in the sunshine hours, and then coast on that stored heat energy in the evenings (again, to the extent comfortable).

It's most straightforward to flatten out the car charging, so I suggest that I get you an initial analysis that does that but leaves the heating alone. And then go from there.

If that sounds good, please give me a sense of how flexible you are with the car charging. ...Typically, how many days per week can the car be at home and plugged in? How important is it that the car be fully charged (or close to fully charged) by the start of the day?

What make / model of car charger do you have? I'll learn about its features... I'm wondering if historical data is available thru it. And some chargers support dynamic charging (using spare PV power).

I'm happy to answer any questions you have about my assumptions and modeling. Or anything else.

Any feedback is appreciated at blog@slowpower.wtf. (I'll assume by default that I have your permission to share your comment here, with you first name and last-name initial.)

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